These release notes cover new developer and user-facing incompatibilities, important issues, features, and major improvements.
HADOOP-15922 | Major | DelegationTokenAuthenticationFilter get wrong doAsUser since it does not decode URL
Fix DelegationTokenAuthentication filter for incorrectly double encode doAs user parameter.
YARN-8761 | Major | Service AM support for decommissioning component instances
Component instance number is not linear increment when decommission feature is used. Application with assumption of linear increment component instance number maybe impacted by introduction of this feature.
NameNodes rely on independent block token key ranges to communicate block token identities to DataNodes and clients in a way that does not create conflicts between the tokens issued by multiple NameNodes. HDFS-6440 introduced the potential for overlaps in key ranges; this fixes the issue by creating 64 possible key ranges that NameNodes assign themselves to, allowing for up to 64 NameNodes to run safely. This limitation only applies within a single Namespace; there may be more than 64 NameNodes total spread among multiple federated Namespaces.
The above patch will resolve the race condition
During a rolling upgrade from Hadoop 2.x to 3.x, NameNode cannot persist erasure coding information, and therefore a user cannot start using erasure coding feature until finalize is done.